This invention pertains to implantable medical devices such as cardiac rhythm management devices. In particular, the invention relates to a method and system incorporated into such a device for gathering clinically useful physiological data.
Implantable cardiac rhythm management devices are commonplace today for the treatment of chronic or recurring cardiac arrhythmias. For example, cardiac pacemakers are implantable devices that replace or supplement a heart""s compromised ability to pace itself (i.e., bradycardia) due to chronotropic incompetence or a conduction system defect by delivering electrical pacing pulses to the heart. Implantable cardioverter/defibrillators (ICD""s) are devices that deliver electrical energy to the heart in order to reverse excessively rapid heart rates (tachycardia) including life threatening cardiac arrhythmias such as ventricular fibrillation. Since some patients have conditions that necessitate pacing and also render them vulnerable to life-threatening arrhythmias, implantable cardiac devices have been developed that combine both functions in a single device.
Cardiac rhythm management devices are typically implanted subcutaneously or submuscularly in a patient""s chest and have leads threaded intravenously into the heart to connect the device to electrodes used for sensing and pacing. Leads may also be positioned on the epicardium by various means. A programmable electronic controller causes shocks to be delivered when fibrillation is detected or pacing pulses to be output in response to lapsed time intervals and sensed electrical activity. Pacemakers sense intrinsic cardiac electrical activity by means of internal electrodes disposed near the chamber to be sensed.
Modern cardiac rhythm management devices also typically have the capability to communicate data via a data link with an external programming device. Such data is transmitted to the pacemaker in order to program its mode of operation as well as define other operating parameters. Data transmitted from the pacemaker can be used to verify the operating parameters as well as relay information regarding the condition of both the pacemaker and the patient. Pacemaker patients are monitored at regular intervals as part of routine patient care and to check the condition of the device. Among the data which may typically be telemetered from the pacemaker are its programming parameters and an electrogram representing the electrical activity of the heart as sensed by the pacemaker.
Pacemakers have also been developed which monitor a patient""s exertion level while the device is functioning in order to adjust the pacing rate. Such devices, referred to as rate-adaptive pacemakers, may use various measurable physiological parameters for this purpose that are related to exertion level including minute ventilation, body activity, electrogram intervals, and body temperature. Because of their continuous access to the patient and their communications capabilities, cardiac rhythm management devices and other similar implantable devices may also offer an ideal platform for gathering and storing clinically useful information which can later be transmitted to an external device.
The present invention is a method and system for monitoring temperature in an implantable medical device such as a cardiac pacemaker or implantable cardioverter/defibrillator. Data from these monitoring operations can then be stored in the memory of the device for later retrieval using an external programmer. In one embodiment, temperature measurements are collected at specified regular intervals to ascertain trends in the patient""s temperature that may be useful in diagnosing certain medical conditions. The collection intervals may be made shorter or longer in order to detect fast or slow trends, respectively. Temperature data may also be collected to reflect average temperature variations over specified periods (e.g., daily) and at specific times of the day. Averages of temperature measurements taken over a period of time can also be used to compensate for component drift in the temperature sensor by calibrating the temperature sensor to match an average temperature which is assumed to be consistently maintained by the body.
In another embodiment, temperature measurements are associated with contemporaneous physiological measurements such as heart rate, respiratory rate, and body activity as well as any device activity to aid the clinician in interpreting the data. Detection of certain events such as arrhythmias or initiation of particular device activities may be used to trigger measurement of temperature and possibly other physiological variables.
Body temperature measurements may be taken using a temperature sensor incorporated into an intravenous lead or otherwise external to the device. Alternatively, temperature sensing circuitry internal to the device housing may be employed to provide the temperature measurement. Because device activity can affect its internal temperature, the system may require that temperature measurements using an internal sensor only be taken during periods in which the device is not actively delivering therapy. In a particular embodiment of an internal sensor, a proportional-to-absolute-temperature (PTAT) current typically used by the device electronics to generate a reference voltage is employed as a temperature sensor. Incorporating a temperature sensor within the device housing allows a clinician to determine if the temperature is within an acceptable range before allowing the device to become operational in a patient and also allows monitoring of temperature before implantation to determine if temperature extremes have occurred during storage which may adversely affect device operation. As the internal temperature of an electronic device may change during certain activities, such as delivery of shock pulses in the case of an implantable cardioverter/defibrillator, either no temperature data is collected during such activities or such data is flagged accordingly.
Another embodiment of the invention involves efficient storage and transmission of temperature data. Because of the limited range of temperature measurements taken within the human body, the range and resolution of stored temperature data can be adjusted so that less storage space is needed and data can be transmitted more efficiently. A non-linear range can also be used so that different resolutions are used with different temperatures.